ANISOTROPY IN LANGMUIR-BLODGETT-FILMS STUDIED BY GENERALIZED SPECTROSCOPIC ELLIPSOMETRY

Langmuir-Blodgett films obtained by successive deposition of a known n umber of fatty-acid monolayers are generally assumed to be uniaxial wi th in-plane isotropy. In order to test the validity of this assumption , we measure the generalized ellipsometric response of such quasi-bidi mensional molecular systems as a function of their azimuthal orientati on theta relative to the incident plane. These measurements which are related to the nan-diagonal terms of the Jones reflection matrix provi de information on the orientation of the principal axes and the princi pal indices of the dielectric tensor of the film. Using a multilayer a nisotropic model based on the Berreman formalism, regression of the ex perimental data gives the following results. The studied LB films are biaxial with one of the principal axis tilted at 1.5 degrees with resp ect to the normal of the film and the in-plane birefringence varies fr om 2 X 10(-2) to 1 X 10(-2) over the 0.24-0.6 mu m spectral domain. Th is in-plane anisotropy is connected to the withdrawing direction of th e substrate during the LB film deposition. (C) 1998 Elsevier Science S .A.